Southampton broke their club record transfer fee on Sunday when they paid Roma £15m, for striker Pablo Osvaldo.
The four-year deal for the 27-year-old Argentina-born Italian international betters the £12.5m Southampton paid for Celtic midfielder Victor Wanyama in July.
He will now be reunited with Saints boss Mauricio Pochettino under whom he played during an 18-month spell with Spanish side Espanyol. Osvaldo, who had also played in Serie A for Atlanta and Fiorentina, should be available for Saturday's English Premier League game against Sunderland at St Mary's.
"Southampton Football Club is delighted to announce the signing of Italy striker Pablo Daniel Osvaldo," a statement on the club's official website said.
Southampton won their opening match of the season at West Bromwich Albion on Saturday, with a late Rickie Lambert penalty securing a 1-0 victory.